This invention relates generally to power tools and, more particularly, to an improved power ring saw.
Power tools are known in which a power unit rotates an annularly shaped tool that is used to cut through or otherwise operate on workpieces. Ring saws are examples of such power tools. One such ring saw is disclosed in U.S. Pat. No. Re. 27,716 to E. Santilli.
In the ring saw disclosed in U.S. Pat. No. Re. 27,716, an annular saw blade is rotatively driven about a virtual axis normal to the plane of the blade by a conical drive roller acting against one of the blade faces. The drive roller is mounted on the shaft of a power unit (e.g., an electric motor), which shaft includes a helical groove that serves to urge the drive roller into frictional engagement with the blade face. The blade has an circular groove formed in one or both of its faces to accommodate conical guide rollers, each of which has a pair of rims formed around its outer surface that engage within the groove and against the inner peripheral edge of the blade. The rims prevent the blade from shifting radially as it rotates about its virtual axis.
Ring saws of the above type can be made in various sizes to perform a wide variety of jobs. The saws are particularly useful where depth of cut is critical because, unlike circular saws in which the saw blade is supported for rotation at its center, ring saws enable a depth of cut greater than the radius of the saw blade.
Ring saws of the type disclosed in the above Santilli patent have experienced various problems. When cutting through a workpiece, the saw blade, which is rotated at high speed, experiences considerable resistance that gives rise to reaction forces tending to shift the virtual axis of the blade radially. These radial forces are resisted primarily by the rims on the conical guide rollers which engage against the inner peripheral edge of the blade and within the circular guide groove. The forces increase the friction between the guide rollers and the blade, causing the blade to heat up and, with time, causing the guide groove in the blade and the rollers to wear. The wearing action gradually loosens the hold on the blade and gives rise to increased blade vibration. Such vibration and heat significantly shorten the useful life of the blade.
U.S. Pat. No. 3,930,310 to E. Santilli discloses an improved version of the ring saw disclosed in the above-cited U.S. Pat. No. Re. 27,716. The improvements are directed to various means to produce stability of the saw blade during cutting. For example, in the improved saw, the conical drive roller axis is canted relative to the blade so that it produces a force on the blade including both a tangential component for rotating the blade and a radial component for counteracting workpiece reaction forces tending to shift the blade radially. Means are also provided to vary the drive roller pressure on the blade, or the angle of the drive roller axis relative to the blade, so as to vary the radial force component in accordance with variations in the reaction forces. Stabilizing means of this type not only add to the overall cost of the saw, but also increase the number of components in the saw that are susceptible to failure. They are also at best only marginally effective in improving blade life.
Experience has shown that, even with the various stabilizing means disclosed in U.S. Pat. No. 3,930,310, ring saw blades used in ring saws of the type disclosed in the above two patents have useful lives only in the range of about 20 minutes to 5 hours of cutting operation. An inspection of the failed blades shows that most such blades fail because cracks develop in the gullets between the saw teeth on the outer periphery of the blade. The failed blades also have a noticeable concavity or "dishing" in the vicinity of the guide groove. Because of the action of the rimmed guide rollers on the blade, the inner annular portion of the blade, that is, the portion of the blade extending radially inwardly from the guide groove to the inner peripheral edge of the blade, is believed to heat up more than the outer peripheral portion of the blade during cutting. Because of this uneven heating, the inner blade portion tends to expand thermally to a greater extent than the outer blade portion. Additionally, the guide groove, which is formed by reducing the effective thickness of the blade, weakens the blade structurally. The combination of the uneven thermal expansion and weakening of the blade in the vicinity of the guide groove are believed to be primarily responsible for the blade cracking, dishing and ultimate failure.